Superplastic Forming (“SPF”) is a material forming process designed for sheet material allowing elongation of several hundred percent. Common materials used with SPF include, but are not limited to, aluminum alloys, titanium alloys, and nickel alloys. The material is heated to promote superplasticity. In a superplastic state, the material is sufficiently pliable so processes that are usually used on plastics may be applied, such as thermoforming, blow forming, vacuum forming and the like.
SPF may be used to create multi-sheet panels (a panel formed of two or more sheets of material). In a two-sheet panel example, two metal sheets are welded together and placed within a molding tool. In a three-sheet panel (a panel formed of a core sheet and two opposing face sheets sandwiching the core sheet) example, the core sheet is welded to the face sheets and placed within the molding tool. When the panel is hot, a pressure is applied, for example, between the two sheets of the two-sheet panel of between the face sheets and the core of the three-sheet panel, and the panel becomes hollow to the form of the molding tool.
Disadvantageously, a multi-sheet panel formed this way has a propensity to exhibit surface imperfections (e.g., “dimpling” or “eyebrowing”) during the SPF process. During the SPF process, pulling stresses applied on a sheet by an adjacent sheet may cause such imperfections. Such imperfections often occur when making three-sheet panels using the SPF process. These imperfections may adversely affect the characteristics (e.g., the aerodynamic characteristics and/or the structural characteristics) of the panel. Thus, current manufacturing techniques, such as SPF, may not be suitable for making panels from three sheets of material.
One alternative used to provide some relief from the formation of surface imperfections (e.g., the dimpling effect) on three-sheet panels is to make the face sheets much thicker than the core sheet. However, this solution comes with an increase in weight and a rather severe performance penalty.
Another alternative is to make a four-sheet panel. The four-sheet panel may not form dimples when the face sheets are forced apart because there are no welds between the core and face sheets. Formation of four-sheet panels combines SPF with a second element, Diffusion Bonding, also known as Superplastic forming and diffusion bonding (“SPF/DB”), to create a panel formed of four sheets of material. Generally, the four metal sheets are welded together (e.g., at their edges) and heated within the confines of a molding tool. When the panel is hot, a pressure is applied between the sheets and the panel becomes hollow to the form of the molding tool. However, since diffusion bonding is used to join the sheets, such bonding may take many hours to form.
Accordingly, three-sheet panels are preferred over four-sheet panels because the three-sheet panels are lighter (three sheets vs. four sheets) and quicker to make (no diffusion bonding). However, three-sheet panels are not currently used because there are no suitable manufacturing techniques for making such panels.
Accordingly, those skilled in the art continue with research and development efforts in the field of three-sheet panel manufacturing.